Fuel composition

ABSTRACT

The present invention provides a fuel composition comprising: (a) a fuel; (b) polyglycerol polyricinoleic acid (c) a monoglyceride of a fatty acid.

The present invention relates to a composition. In particular the present invention relates to fuel compositions having reduced nitrogen oxide emissions when combusted.

As discussed in U.S. Pat. No. 7,491,247 environmental considerations and government regulations have increased the need to reduce nitrogen oxide (NOx) production. Nitrogen oxides comprise a major irritant in smog and are believed to contribute to tropospheric ozone which is a known threat to health. Relatively high flame temperatures reached in internal combustion engines, for example diesel-fuelled engines, increase the tendency for the production of nitrogen oxides (NOx). These are formed from both the combination of nitrogen and oxygen in the combustion chamber and from the oxidation of organic nitrogen species in the fuel.

Various methods for reducing NOx production include the use of catalytic converters, engine timing changes, exhaust recirculation, and the burning of “clean” fuels. These methods are generally too expensive and/or too complicated to be placed in widespread use. The rates at which NOx are formed is related to the flame temperature; a small reduction in flame temperature can result in a large reduction in the production of nitrogen oxides.

It has been shown that introducing water into the combustion zone can lower the flame temperature and thus lower NOx production, however; the direct injection of water requires costly and complicated changes in engine design. Further attempts to use water to reduce flame temperature include the use of aqueous fuels, i.e., incorporating both water and fuel into an emulsion. Problems that may occur from long-term use of aqueous fuels include precipitate depositions from coalescing ionic species resulting in filter plugging and inorganic post combustion deposits resulting in turbo fouling. Another problem related to aqueous fuel compositions is that they often require substantial engine modifications, such as the addition of in-line homogenizers, thereby limiting their commercial utility.

Another method for introducing water into the combustion area is to use fuel emulsions in which water is emulsified into a fuel continuous phase, i.e., invert fuel emulsions. A problem with these invert fuel emulsions is obtaining and maintaining the stability of the emulsion under conventional use conditions. Gravitational phase separation (during storage) and high temperature high pressure/shear flow rate phase separation (in a working engine) of these emulsions present the major hurdle preventing their commercial use.

The present invention addresses the problems associated with the use of fuel emulsion compositions by providing a stable fuel emulsion composition with the beneficial reduction in NOx emissions.

The present invention alleviates the problems of the prior art.

In one aspect the present invention provides a fuel composition comprising:

(a) a fuel; (b) polyglycerol polyricinoleic acid and (c) a monoglyceride of a fatty acid.

In one aspect the present invention provides a method for improving the stability of a fuel composition containing fuel and water, the method comprising mixing with the fuel and water, (a) polyglycerol polyricinoleic acid; and (b) a monoglyceride of a fatty acid.

In one aspect the present invention provides use of polyglycerol polyricinoleic acid and a monoglyceride of a fatty acid for improving the stability of a fuel composition containing fuel and water.

In one aspect the present invention provides a kit for preparing a fuel composition as defined herein, the kit comprising (a) polyglycerol polyricinoleic acid and (b) a monoglyceride of a fatty acid, in separate packages or containers, or combined in a single package or container; together with instructions for use to prepare the fuel composition.

We have shown that when each a polyglycerol polyricinoleic acid or a monoglyceride of a fatty acid are used alone as an emulsifier in a fuel containing water, that single emulsifier fails to provide a fuel/water emulsion which is stable during storage. In contrast we have surprisingly found that the specific combination of these two emulsifiers, namely a polyglycerol polyricinoleic acid and a monoglyceride of a fatty acid provides fuel and water emulsions which are stable at least with regard to separation until the combustion of the fuel.

For ease of reference these and further aspects of the present invention are now discussed under appropriate section headings. However, the teachings under each section are not necessarily limited to each particular section.

Composition

As previously mentioned, in one aspect the present invention provides a fuel composition comprising: (a) a fuel; (b) polyglycerol polyricinoleic acid and (c) a monoglyceride of a fatty acid.

Polyglycerol Polyricinoleic Acid

As is understood by one skilled in the art polyglycerol polyricinoleic acid is an emulsifier comprising a polyglycerol ‘backbone’ onto which ricinoleic acid side chains are attached. Ricinoleic acid ((9Z,12R)-12-Hydroxyoctadec-9-enoic acid) has hydroxy group at the 12 position onto which further ricinoleic side chains may be attached.

The polyglycerol may be of any suitable length. In one aspect the polyglycerol comprises one or a mixture of more than one of the polyglycerols selected from the group consisting of diglycerol, triglycerol, tetraglycerol, pentaglycerol, hexaglycerol, heptaglycerol, octaglycerol, nonaglycerol and decaglycerol. In one aspect the polyglycerol comprises one or a mixture of more than one of the polyglycerols selected from the group consisting of diglycerol, triglycerol and tetraglycerol.

The ricinoleic acid side chains, namely the polyricinoleic acid, attached to the polyglycerol may be of any suitable length. In one aspect the fatty acid polymerised chain length of the polyricinoleic acid is from 1 to 10. In another aspect the fatty acid polymerised length of the polyricinoleic acid is from 4 to 6.

The ricinoleic acid may be provided from any suitable source. Thus in one aspect, the polyglycerol polyricinoleic acid is prepared from hydroxy fatty acids of hydrogenated or non-hydrogenated castor oil.

In one aspect the polyglycerol polyricinoleic acid has a hydroxyl value of about 20 to about 120 mgKOH.

In a further and preferred aspect the polyglycerol polyricinoleic acid has at least one of the following characteristics:

-   -   i) an acid value of less than or equal to 10.0 mg KOH;     -   ii) a hydroxyl value of about 80.0 to about 100.0 mgKOH;     -   iii) an iodine value of about 72 to about 103 g I₂.

Preferably the polyglycerol polyricinoleic acid has more than one of the characteristics i) to iii). More preferably the polyglycerol polyricinoleic acid has all of the characteristics i) to iii).

Monoglyceride of a Fatty Acid

The monoglyceride of a fatty acid may be a monoglyceride having any suitable fatty chain lengths. The monoglyceride of a fatty acid may be a monoglyceride of a single fatty acid, or monoglycerides of a mixture of fatty acids. The fatty chain lengths of the monoglycerides in a mixture of monoglycerides need not be of the same length. Typically the monoglyceride of a fatty acid is monoglyceride of a fatty acid of a C12 to C22 fatty acid. Preferably the monoglyceride of a fatty acid is monoglyceride of a C16 or C22 fatty acid. Preferably the monoglyceride of a fatty acid is monoglyceride of a C16 or C18 fatty acid.

The fatty acid of the monoglyceride of a fatty acid may be saturated fatty acid, unsaturated fatty acid or a mixture of saturated fatty acid and unsaturated fatty acid. In one aspect the monoglyceride of a fatty acid is monoglyceride of unsaturated fatty acid. Preferably the monoglyceride of a fatty acid is a monoglyceride of mono or di unsaturated fatty acid.

Preferred monoglycerides of a fatty acid may be selected from

-   -   monoglyceride of saturated C16 fatty acid     -   monoglyceride of saturated C18 fatty acid     -   monoglyceride of mono unsaturated C18 fatty acid     -   monoglyceride of di unsaturated C18 fatty acid     -   monoglyceride of saturated C16 fatty acid; and monoglyceride of         saturated C18 fatty acid     -   monoglyceride of saturated C16 fatty acid; and monoglyceride of         mono unsaturated C18 fatty acid     -   monoglyceride of saturated C16 fatty acid; and monoglyceride of         di unsaturated C18 fatty acid     -   monoglyceride of saturated C18 fatty acid; and monoglyceride of         mono unsaturated C18 fatty acid     -   monoglyceride of saturated C18 fatty acid; and monoglyceride of         di unsaturated C18 fatty acid     -   monoglyceride of mono unsaturated C18 fatty acid; and         monoglyceride of di unsaturated C18 fatty acid     -   monoglyceride of saturated C16 fatty acid; and monoglyceride of         saturated C18 fatty acid; and monoglyceride of mono unsaturated         C18 fatty acid     -   monoglyceride of saturated C16 fatty acid; and monoglyceride of         saturated C18 fatty acid; and monoglyceride of di unsaturated         C18 fatty acid     -   monoglyceride of saturated C16 fatty acid; and monoglyceride of         mono unsaturated C18 fatty acid; and monoglyceride of di         unsaturated C18 fatty acid     -   monoglyceride of saturated C18 fatty acid; and monoglyceride of         mono unsaturated C18 fatty acid; and monoglyceride of di         unsaturated C18 fatty acid     -   monoglyceride of saturated C16 fatty acid; and monoglyceride of         saturated C18 fatty acid; and monoglyceride of mono unsaturated         C18 fatty acid; and monoglyceride of di unsaturated C18 fatty         acid

Preferred monoglycerides of a fatty acid may be selected from

-   -   palmitic acid monoglyceride     -   stearic acid monoglyceride     -   oleic acid monoglyceride     -   linoleic acid monoglyceride     -   palmitic acid monoglyceride; and stearic acid monoglyceride     -   palmitic acid monoglyceride; and oleic acid monoglyceride     -   palmitic acid monoglyceride; and linoleic acid monoglyceride     -   stearic acid monoglyceride; and oleic acid monoglyceride     -   stearic acid monoglyceride; and linoleic acid monoglyceride     -   oleic acid monoglyceride; and linoleic acid monoglyceride     -   palmitic acid monoglyceride; and stearic acid monoglyceride; and         oleic acid monoglyceride     -   palmitic acid monoglyceride; and stearic acid monoglyceride; and         linoleic acid monoglyceride     -   palmitic acid monoglyceride; and oleic acid monoglyceride; and         linoleic acid monoglyceride     -   stearic acid monoglyceride; and oleic acid monoglyceride; and         linoleic acid monoglyceride     -   palmitic acid monoglyceride; and stearic acid monoglyceride; and         oleic acid monoglyceride; and linoleic acid monoglyceride

As discussed above, each of the polyglycerol polyricinoleic acid and the monoglyceride of a fatty acid alone do not provide a stable fuel and water emulsion. Therefore each of these components must be present in sufficient amounts, in relative and absolute terms, to provide a stable emulsion. The emulsion must be stable such that, in use, the water and the fuel do not separate. In use the emulsion is typically formed shortly before it is required for combustion. This is performed by combination of the essential materials, namely the polyglycerol polyricinoleic acid, the monoglyceride of a fatty acid, the fuel and the water. The emulsion is then fed into the fuel delivery system to be combusted. Between formation of the fuel emulsion and its eventual combustion, the emulsion should not separate. This period between formation and combustion may be relatively short if the emulsion is combusted almost immediately. However, in a number of circumstances the period may be longer. Examples of such circumstances include fuel delivery systems in which a proportion of the fuel is combusted and the remainder of the fuel is recirculated around the fuel delivery system. This is common in diesel and marine gasoil engines. Further circumstances are where an engine is shut down either completely or partially (by shut down of one or more cylinders of a multi-cylinder engine). During the period of shut down it is a requirement that the fuel emulsion should not separate. If separation were to occur, restarting of the engine or of the inactive cylinder(s) may not be possible. Periods of stability required by many industries are at least 1 hour, such as at least 2 hours, such as at least 3 hours.

In one aspect the present invention provides a fuel composition comprising:

(a) a fuel; (b) polyglycerol polyricinoleic acid; (c) a monoglyceride of a fatty acid; and (d) water, wherein the fuel composition is an emulsion and wherein the emulsion is stable with regard to separation of the emulsion for a period of at least 1 hour after the formation of the emulsion. Preferably the emulsion is stable with regard to separation of the emulsion for a period of at least 2 hours after the formation of the emulsion. Preferably the emulsion is stable with regard to separation of the emulsion for a period of at least 3 hours after the formation of the emulsion.

In one aspect the ratio of (b) polyglycerol polyricinoleic acid to (c) a monoglyceride of a fatty acid is from 0.9:0.1 to 0.1:0.9. In one aspect the ratio of (b) polyglycerol polyricinoleic acid to (c) a monoglyceride of a fatty acid is from 0.7:0.3 to 0.1:0.9.

We have further found that although a broad range of ratios provides advantages over the prior art systems, at a specific range of ratios, particularly strong stability is observed. In one preferred aspect the ratio of (b) polyglycerol polyricinoleic acid to (c) a monoglyceride of a fatty acid is from 0.625:0375 to 0.125:0.875. In one highly preferred aspect, the monoglyceride of a fatty acid is monoglyceride of a C16 or C18 fatty acid, the polyglycerol polyricinoleic acid is prepared from hydroxy fatty acids of hydrogenated or non-hydrogenated castor oil wherein the polyglycerol comprises a mixture of polyglycerols selected from the group consisting of diglycerol, triglycerol and tetraglycerol; and the ratio of (b) polyglycerol polyricinoleic acid to (c) a monoglyceride of a fatty acid is from 0.625:0.375 to 0.125:0.875.

When a mixture of (b) polyglycerol polyricinoleic acid and (c) a monoglyceride of a fatty acid is provided in accordance with the present invention, the mixture may be dosed in the water and fuel composition in any suitable amount to provide an emulsion of desired stability. In one aspect the fuel composition comprises (b) polyglycerol polyricinoleic acid and (c) a monoglyceride of a fatty acid in a total combined amount of from 0.1 to 2.0 wt % based on the total fuel composition. In a further aspect the fuel composition comprises (b) polyglycerol polyricinoleic acid and (c) a monoglyceride of a fatty acid in a total combined amount of from 0.1 to 1.0 wt % based on the total fuel composition. In a further aspect the fuel composition comprises (b) polyglycerol polyricinoleic acid and (c) a monoglyceride of a fatty acid in a total combined amount of from 0.5 to 1.0 wt % based on the total fuel composition.

The polyglycerol polyricinoleic acid is dosed in the water and fuel composition in any suitable amount to provide an emulsion of desired stability. In one aspect the fuel composition comprises polyglycerol polyricinoleic acid in an amount of from 0.05 to 2.0 wt % based on the total fuel composition. In a further aspect the fuel composition comprises polyglycerol polyricinoleic acid in an amount of from 0.05 to 1.0 wt % based on the total fuel composition. In a further aspect the fuel composition comprises polyglycerol polyricinoleic acid in an amount of from 005 to 0.8 wt % based on the total fuel composition. In a further aspect the fuel composition comprises polyglycerol polyricinoleic acid in an amount of from 0.1 to 0.8 wt % based on the total fuel composition. In a further aspect the fuel composition comprises polyglycerol polyricinoleic acid in an amount of from 0.1 to 0.7 wt % based on the total fuel composition. In a further aspect the fuel composition comprises polyglycerol polyricinoleic acid in an amount of from 0.125 to 0.625 wt % based on the total fuel composition.

The monoglyceride of a fatty acid is dosed in the water and fuel composition in any suitable amount to provide an emulsion of desired stability. In one aspect the fuel composition comprises a monoglyceride of a fatty acid in an amount of from 0.05 to 1.0 wt % based on the total fuel composition. In a further aspect the fuel composition comprises a monoglyceride of a fatty acid in an amount of from 0.1 to 1.0 wt % based on the total fuel composition. In a further aspect the fuel composition comprises a monoglyceride of a fatty acid in an amount of from 0.2 to 1.0 wt % based on the total fuel composition. In a further aspect the fuel composition comprises a monoglyceride of a fatty acid in an amount of from 0.3 to 1.0 wt % based on the total fuel composition. In a further aspect the fuel composition comprises a monoglyceride of a fatty acid in an amount of from 0.375 to 0.875 wt % based on the total fuel composition.

It is understood by one skilled in the art that monoglycerides of fatty acids by the nature of their preparation are typically supplied as a mixture of monoglycerides of a fatty acid and diglycerides of a fatty acid. Such mixtures are referred to by those skilled in the art as a mono-diglyceride of a fatty acid. In one aspect, the monoglycerides of fatty acids for use in the present invention are provided in a mixture of monoglyceride of a fatty acid and diglyceride of a fatty acid, namely as a mono-diglyceride of a fatty acid. Thus the present invention provides

-   -   a fuel composition comprising:     -   (a) a fuel; (b) polyglycerol polyricinoleic acid (c) a mixture         of monoglyceride of a fatty acid and diglyceride of a fatty         acid.     -   a method for improving the stability of a fuel composition         containing fuel and water, the method comprising mixing with the         fuel and water, (a) polyglycerol polyricinoleic acid; and (b) a         mixture of monoglyceride of a fatty acid and diglyceride of a         fatty acid     -   use of polyglycerol polyricinoleic acid and a mixture of         monoglyceride of a fatty acid and diglyceride of fatty acid for         improving the stability of a fuel composition containing fuel         and water.

The mixture of monoglycerides of fatty acids and diglycerides of fatty acids may be a distilled product or a non-distilled product, in a preferred aspect, the monoglyceride of a fatty acid is a distilled monoglyceride of a fatty acid.

Fuel

As discussed herein, the emulsifiers described allow for the preparation of an emulsion of fuel and water. A fuel suitable for preparing into an emulsion but which has yet to be combined with water is hereby encompassed within the present invention. However, in a preferred aspect, the fuel containing the emulsifiers is combined with water and the fuel composition further comprises (d) water. It will be appreciated that in this aspect the fuel composition may be prepared by first dosing the emulsifiers (polyglycerol polyricinoleic acid and monoglyceride of a fatty acid) into the fuel, such as marine gasoil (MGO), after which water is dosed into the fuel/emulsifier blend.

The amount of water may be selected based on the requirements of the combustion system, in one aspect the fuel composition further comprises (d) water in an amount of from 10 to 70 wt % based on the total fuel composition. Preferably the water is present in an amount of from 30 to 60 wt % based on the total fuel composition. Preferably the water is present in an amount of from 33 to 50 wt % based on the total fuel composition.

The composition according to the present invention may comprise one or more additives for example, to improve various aspects of the fuel to which the composition is typically added or to improve various aspects of the combustion system performance. Suitable additional additives include detergents, carrier oils, anti-oxidants, corrosion inhibitors, colour stabilisers, metal deactivators, cetane number improvers, other combustion improvers, antifoams, pour point depressants, cold filter plugging depressants, wax anti-settling additives, dispersants, deodorants, dyes, smoke suppressants, lubricity agents, and other particulate filter regeneration additives. However, in one aspect the fuel composition comprises glycerol in an amount of less than 0.1 wt % based on the total fuel composition, such as in an amount of less than 0.05 wt % based on the total fuel composition, such as in an amount of less than 0.02 wt % based on the total fuel composition, such as in an amount of less than 0.01 wt % based on the total fuel composition, such as in an amount of less than 0.005 wt % based on the total fuel composition, such as in an amount of less than 0.001 wt % based on the total fuel composition.

The fuel may be any fuel suitable for combustion where reduction of NOx is desired. In one aspect the fuel is a fuel for spark ignition engines such as a gasoline engine. Preferably the fuel is a fuel for a high compression spontaneous ignition engine. In one aspect the fuel is selected from diesel, heavy fuel oil, marine gasoil (MGO) and kerosene. The diesel may be biodiesel, low sulphur diesel and ultra-low sulphur diesel. Preferably the fuel is marine gasoil. The marine gasoil may be any suitable marine gasoil. In one aspect it is a fuel having a (i) a density of 0.85-0.89 g/cm³, a cetane Number of approximately 45; and a flash point of greater than 55° C.

Kit

As discussed herein, in one aspect the present invention provides a kit for preparing a fuel composition as defined herein, the kit comprising (a) polyglycerol polyricinoleic acid; and (b) a monoglyceride of a fatty acid, in separate packages or containers, or combined in a single package or container; together with instructions for use to prepare the fuel composition.

In one aspect (a) polyglycerol polyricinoleic acid and (b) a monoglyceride of a fatty acid of a fatty acid are provided in separate packages or containers. In one aspect (a) polyglycerol polyricinoleic acid and (b) a monoglyceride of a fatty acid of a fatty acid are provided combined in a single package or container.

Aspects of the invention are defined in the appended claims.

The present invention will now be described in further detail in the following examples.

EXAMPLES

As discussed herein addition of water to fuels such as diesel can reduce NOx pollution, for example and particularly from ships. The presence of water reduces the combustion temperature in the engine resulting in less NOx formation. It is understood that up to 50% water addition may be required to obey future limits on maximum NOX emission set for the future.

Typically on ship use of fuel and water emulsions is achieved by preparation on board of emulsions. Thus the emulsions require only 1-3 hours stability

In the following examples polyglycerol polyricinolate and distilled monoglycerides were tested as emulsifiers for water-in-fuel emulsions. As discussed herein, it was surprisingly found that both emulsifiers when used alone failed in stabilising the water-in-fuel emulsions, whereas the two emulsifiers in combination created stable emulsions. The stable emulsion did not undergo sedimentation of the water droplets during a 3 hour test period.

GRINDSTED PGPR 90 a polyglycerol polyricinolate, and specifically a polyglycerol ester of polycondensed fatty acids from castor oil. GRINDSTED PGPR 90 is available from Danisco A/S, Denmark. GRINDSTED PGPR 90 has i) an acid value of less than or equal to 6 mg KOH; ii) a hydroxyl value of 80 to 100 mgKOH; and iii) an iodine value of 72 to 103 g I₂.

DIMODAN U/J a distilled monoglyceride, and specifically a distilled monoglyceride made from refined sunflower oil. DIMODAN U/J is available from Danisco A/S.

GRINDSTED PGPR 90 and DIMODAN U/J were tested at dosages ranging from 0.6%-1.0% based on the total emulsion. The tests were performed at water contents of 33% and 50% at temperatures of 40° C. and 55° C.

Method

Test conditions: temperature range 40′C-55° C. and water content 33%-50% based on the emulsion.

Emulsions were characterised in respect to emulsion stability (phase separation and sedimentation), water droplet size distribution (droplet size by NMR and CLSM) and viscosity from flow curves.

Preparation of Emulsions:

The emulsifiers were dissolved in 40/55° C. MSO and 40/55° C. water was added to the Marine Gasoil (MGO) during high speed mixing with Ultra Turrax at 20500 rpm for 64 sec as standard.

The emulsions were investigated according to below described methods and subsequently stored at 40/55° C.

Emulsions Stability

The emulsions were evaluated visually for stability. Two phenomena were evaluated: water droplet sedimentation due to gravity force and water separation due to coalescence. The samples were monitored for 3 hours.

Microscopy:

CLSM (Confocal Laser Scanning Microscope—Leica TCS SP2) using Nile Red and FITC for MGO and water droplets staining, respectively.

Water Droplet Size Distribution:

The water droplet size distribution was measured by NMR (Bruker Minispec mq20 NMR Analyzer) based on log-normal size distribution.

Bulk Rheology

A flow curve was measured for selected emulsions just after preparation in the shear rate range 10-2000 l/s using Physica Rheoplus, measuring system DG26.7-SN12751; d=0 mm,

Results and Discussion

The results show that mixing ratios of PGPR90:DIMODAN U/J in the range 62.5:37.5 to 12.5:87.5 prevented sedimentation and water phase separation throughout the 3 hours test phase. Ratios outside above mixing ratios resulted in some sedimentation but less than compared to each of the emulsifiers used alone.

Test PGPR 90 and DIMODAN U/J at the ratios shown below and a total dosage of 1% in MGO Shell containing 33% water. The emulsifiers were added and dissolved into the MGO at 40° C. 40° C. was maintained during mixing with Ultra Turrax at 20500 rpm for 64 seconds. The results are shown in Table 1.

TABLE 1 Temp Trial PGPR 90 % U/J % Water % MGO ° C. D50.0 μm 1 1 0 33 Shell 40 10.3 2 0.875 0.125 33 Shell 40 9.9 3 0.750 0.250 33 Shell 40 10.6 4 0.625 0.375 33 Shell 40 11.5 5 0.500 0.500 33 Shell 40 11.5 6 0.375 0.625 33 Shell 40 10.4 7 0.250 0.750 33 Shell 40 9.9 8 0.125 0.875 33 Shell 40 9.9 9 0 1 33 Shell 40 10.7

The droplet size distribution in table 1 shows a nearly constant D50.0 independently of the mixing ratio between PGPR 90 and DIMODAN U/J.

In FIG. 1 the flow curves shows that emulsions with PGPR 90 behave nearly Newtonian in the complete shear rate range in contrast to emulsions with only distilled monoglycerides added. The Newtonian viscosity was approx. 7 mPa s

FIG. 2 shows images of the samples of Table 1. For each composition, images were recorded at 1, 2 and 3 hours after mixing. FIG. 3 shows graphically the degree of sedimentation expressed as amount of free MGO on top of the samples. The graph was constructed by measuring and calculating the height of the free oil layer in percentage of the total emulsion height by image analysis. Five samples in the mid-range perform excellently with no sedimentation within the first 3 hours Samples with only PGPR 90 or DIMODAN U/J had severe sedimentation

FIG. 2 and FIG. 3 clearly show, that PGPR:DIMODAN in the mixing ratios 62.5:37.5:12.5:87.5 provide superior stability with sedimentation during the 3 hours seen as free gasoil in top of the samples.

The confocal laser scanning microscopy (CLSM) images of FIG. 4 taken just after emulsification did not show any major difference and confirmed that the water droplets were at the same size as seen by NMR.

Test 2—PGPR 90 and DIMODAN U/J at the amounts shown below and at the total dosage shown below were mixed in MGO Shell containing 33% or 50% water. The emulsifiers were added and dissolved into the MGO at 40° C. or 55° C. and this temperature was maintained during mixing with Ultra Turrax at 20500 rpm for 64 seconds. The results are shown in Table 2.

TABLE 2 PGPR + Temp Trial PGPR 90 % U/J % U/J % Water % ° C. D50.0 μm 11 0.3 0.3 A 0.6 50 40 5.2 12 0.075 0.525 B 0.6 50 40 4.7 13 0.5 0.5 A 1.0 33 55 11.3 14 0.125 0.875 B 1.0 33 55 9.8 15 0.3 0.3 A 0.6 50 55 5.2 16 0.075 0.525 B 0.6 50 55 4.5

FIG. 6 shows images of the samples of Table 2. For each composition, images were recorded at 1, 2 and 3 hours after mixing.

The droplet size distribution shows that high water content results in smaller droplets. The larger droplets for sample 13 and 14 was also reflected in the CLSM images of FIG. 7, where samples 13 and 14 showed less smooth structure as compared to the rest of the samples.

The rheology was controlled by temperature and water content. FIG. 5 demonstrates that shear thinning was only noticed at 55% water level and only at very low shear rates. Newtonian flow was obtained already at approx. 100 l/s. There was no difference between emulsifier mixtures A and B in respect to viscosity.

All publications mentioned in the above specification are herein incorporated by reference. Various modifications and variations of the described methods and system of the invention will be apparent to those skilled in the art without departing from the scope and spirit of the invention. Although the invention has been described in connection with specific preferred embodiments, it should be understood that the invention as claimed should not be unduly limited to such specific embodiments. Indeed, various modifications of the described modes for carrying out the invention which are obvious to those skilled in chemistry or related fields are intended to be within the scope of the following claims 

1. A fuel composition comprising: (a) a fuel; (b) polyglycerol polyricinoleic acid; and (c) a monoglyceride of a fatty acid.
 2. A fuel composition according to claim 1 wherein the fuel is selected from diesel, heavy fuel oil, marine gasoil and kerosene.
 3. A fuel composition according to claim 1 or 2 wherein the fuel is diesel.
 4. A fuel composition according to any one of the preceding claims wherein the polyglycerol comprises one or a mixture of more than one of the polyglycerols selected from the group consisting of diglycerol, triglycerol, tetraglycerol, pentaglycerol, hexaglycerol, heptaglycerol, octaglycerol, nonaglycerol and decaglycerol.
 5. A fuel composition according to any one of the preceding claims wherein the fatty acid polymerised chain length of the polyricinoleic acid is from 1 to
 10. 6. A fuel composition according to any one of the preceding claims wherein the polyglycerol polyricinoleic acid is prepared from hydroxy fatty acids of hydrogenated or non-hydrogenated castor oil.
 7. A fuel composition according to any one of the preceding claims, wherein the polyglycerol polyricinoleic acid has a hydroxyl value of about 20 to about 120 mgKOH.
 8. A fuel composition according to any one of the preceding claims, wherein the monoglyceride of a fatty acid is monoglyceride of a fatty acid of a C16 to C22 fatty acid.
 9. A fuel composition according to claim 8, wherein the monoglyceride of a fatty acid is monoglyceride of a C16 or C18 fatty acid.
 10. A fuel composition according to any one of the preceding claims, wherein the monoglyceride of a fatty acid is monoglyceride of unsaturated fatty acid.
 11. A fuel composition according to any one of the preceding claims, wherein the monoglyceride of a fatty acid is monoglyceride of mono or di unsaturated fatty acid.
 12. A fuel composition according to any one of the preceding claims, wherein the monoglyceride of a fatty acid is a mixture of 1) monoglyceride of saturated C16 fatty acid 2) monoglyceride of saturated C18 fatty acid 3) monoglyceride of mono unsaturated C18 fatty acid; and 4) monoglyceride of di unsaturated C18 fatty acid.
 13. A fuel composition according to any one of the preceding claims, wherein the monoglyceride of a fatty acid is a mixture of 1) palmitic acid monoglyceride 2) stearic acid monoglyceride 3) oleic acid monoglyceride; and 4) linoleic acid monoglyceride.
 14. A fuel composition according to any one of the preceding claims, wherein the ratio of (b) polyglycerol polyricinoleic acid to (c) a monoglyceride of a fatty acid is from 0.7:0.3 to 0.1:0.9.
 15. A fuel composition according to any one of the preceding claims, wherein the ratio of (b) polyglycerol polyricinoleic acid to (c) a monoglyceride of a fatty acid is from 0.625:0.375 to 0.125:0.875.
 16. A fuel composition according to any one of the preceding claims, wherein the fuel composition comprises (b) polyglycerol polyricinoleic acid and (c) a monoglyceride of a fatty acid in a total combined amount of from 0.1 to 2.0 wt % based on the total fuel composition.
 17. A fuel composition according to any one of the preceding claims, wherein the fuel composition comprises (b) polyglycerol polyricinoleic acid and (c) a monoglyceride of a fatty acid in a total combined amount of from 0.1 to 1.0 wt % based on the total fuel composition.
 18. A fuel composition according to any one of the preceding claims, wherein the fuel composition comprises (b) polyglycerol polyricinoleic acid and (c) a monoglyceride of a fatty acid in a total combined amount of from 0.5 to 1.0 wt % based on the total fuel composition.
 19. A fuel composition according to any one of the preceding claims, wherein the fuel composition further comprises (d) water.
 20. A fuel composition according to claim 19, wherein the fuel composition further comprises (d) water in an amount of from 10 to 70 wt % based on the total fuel composition.
 21. A fuel composition according to claim 19, wherein the fuel composition further comprises (d) water in an amount of from 30 to 60 wt % based on the total fuel composition.
 22. A fuel composition according to claim 19, wherein the fuel composition further comprises (d) water in an amount of from 33 to 50 wt % based on the total fuel composition.
 23. A fuel composition according to any one of the preceding claims, wherein the fuel composition comprises glycerol in an amount of less than 0.1 wt % based on the total fuel composition.
 24. A method for improving the stability of a fuel composition containing fuel and water, the method comprising mixing with the fuel and water, (b) polyglycerol polyricinoleic acid; and (c) a monoglyceride of a fatty acid.
 25. A method according to claim 24, characterised by the features of any one of claims 2 to
 19. 26. Use of polyglycerol polyricinoleic acid and a monoglyceride of a fatty acid for improving the stability of a fuel composition containing fuel and water.
 27. A kit for preparing a fuel composition as defined in any one of claims 1 to 23, the kit comprising (a) polyglycerol polyricinoleic acid; and (b) a monoglyceride of a fatty acid, in separate packages or containers, or combined in a single package or container; together with instructions for use to prepare the fuel composition.
 28. A fuel composition substantially as hereinbefore described with reference to any one of the Examples.
 29. A method substantially as hereinbefore described with reference to any one of the Examples.
 30. A use substantially as hereinbefore described with reference to any one of the Examples.
 31. A kit substantially as hereinbefore described with reference to any one of the Examples. 